The present invention relates to a ridge-waveguide type semiconductor laser device, and particularly to a ridge-waveguide type semiconductor laser device having a large half-width value θpara of a far-field pattern (FFP) in a direction horizontal to a hetero-interface, and having a desired laser characteristic at the time of operation with a high power.
In semiconductor laser devices including long-wavelength GaAs or INP based semiconductor laser devices and short-wavelength nitride based III-V group compound semiconductor laser devices, a ridge-waveguide type semiconductor laser device has been often used in various applications for a reason of easy fabrication and the like.
A ridge-waveguide type semiconductor laser is one of index guided types configured such that an upper portion of an upper cladding layer and a contact layer are formed into a stripe-shaped ridge, and both sides of the ridge and portions, located on both sides of the ridge, of the upper cladding layer are covered with an insulating layer to form a current construction layer and also an effective refractive index difference is provided in the lateral direction, whereby a mode control is performed.
A configuration of a short-wavelength ridge-waveguide type nitride based III-V group compound semiconductor laser device (hereinafter, referred to as “nitride based semiconductor laser device”) will be described with reference to FIG. 4. FIG. 4 is a sectional view showing a configuration of a nitride based semiconductor laser device.
Referring to FIG. 4, a nitride based semiconductor laser device 10 basically has a stacked structure in which a plurality of layers are stacked on a sapphire substrate 12 via a GaN buffer layer (not shown). The plurality of layers stacked on the sapphire substrate 12 are an n-GaN contact layer 14, an N—AlGaN (content of A1:8%) cladding layer 16 having a thickness of 1.0 μm, an n-GaN optical guide layer 18 having a thickness of 0.1 μm, an MQW (Multiple Quantum Well) active layer 20 of three well layers, a p-GaN optical guide layer 22 having a thickness of 0.1 μm, a p-(GaN:Mg/AlGaN)-SLS (strained-layer superlattice) cladding layer 24, and a p-GaN contact layer 26 having a thickness of 0.1 μm.
In this stacked structure, an upper portion of the p-cladding layer 24 and the p-contact layer 26 are formed as a stripe-shaped ridge 28. An upper portion of the n-contact layer 14, the n-cladding layer 16, the n-optical guide layer 18, the MQW active layer 20, the p-optical guide layer 22, and remaining layer portions 24a of the p-cladding layer 24 are formed as a mesa structure extending in the same direction as the extending direction of the ridge 28.
A ridge width W of the ridge 28 is typically set to 1.6 μm, a ridge height H is typically set to 0.6 μm, and a thickness T of each of the remaining layer portions 24a, located on both sides of the ridge 28, of the p-cladding layer 24 is typically set to 0.15 μm.
An insulating film 30 composed of an SiO2 film is formed on both side surfaces of the ridge 28 and the remaining layer portions 24a, located on both the sides of the ridge 28, of the p-cladding layer 24.
A p-side electrode 32 composed of a multi-layer metal film made from Pd/Pt/Au is formed on the insulating film 30 in such a manner as to be brought into contact with the p-contact layer 26 via a window formed in the insulating film 30. An n-side electrode 34 composed of a multi-layer metal film made from Ti/Pt/Au is formed on the n-contact layer 14.
By the way, along with the expanded applications of nitride based semiconductor laser devices, it has been required to increase a half-value width (hereinafter, referred to as “θpara”) of a far-field pattern (FFP) in the direction being horizontal to a hetero interface of a resonance structure, and to keep a desired optical power-injected current characteristic up to a high power region by increasing a kink level.
For example, when used as a light source of an optical pickup, a nitride based semiconductor laser device has been required to have the half-value width θpara as large as 7° or more and a kink level as high as about 60 mW.
However, in the case of setting structure factors, such as a ridge width or a thickness of a remaining layer portion of an upper cladding layer, of a nitride based semiconductor laser device, any design criterion being necessary and sufficient to meet the above-described strict requirement has not been established.
For example, since a design range of a nitride based semiconductor laser device is very narrow, if the half-value width θpara of the far-field pattern (FFP) for an elliptic beam in a direction parallel to the hetero interface is set to 7° or more, then the kink characteristic may be degraded. Accordingly, it becomes very important to clarify such a design range.
While the problem of the related art has been described by example of a nitride based semiconductor laser device, a long-wavelength ridge-waveguide type semiconductor laser device, which is longer in oscillation wavelength than the nitride based semiconductor laser device, for example, a GaAs or InP based ridge-waveguide type semiconductor laser device has the same problem.